1. Field of the Invention
This invention relates generally to industrial type burner units and more particularly methods of and apparatus for supplying and mixing fuel and combustion air within such burner units.
2. Discussion of the Prior Art
Industrial drying operations, for example, aggregate heating and drying operations of hot asphalt material production processes make use of high capacity burner units. State of the art burner units are capable of generating 250 million or more BTU (British Thermal Units) per hour. These high capacity burner units are typically operated with industrial grades of fuels, such as a burner grade diesel oil or special heating oils which may be heavier and more viscous than diesel oil. Dispersing fuel into combustion air and mixing the liquid fuel with combustion air to obtain quickly a most uniform mixture has been a problem over the years. Much development work has been applied to improve on fuel and air mixing devices and to thereby improve on the efficiencies of burner units. It is generally recognized that gaseous fuels are more readily mixed with combustion air than liquid fuels. Liquid fuels typically need to be first "atomized", meaning they are mixed with high pressure air and the high pressure air and liquid fuel mixture is released in a typically conical configuration as a spray of fine fuel droplets.
A problem occurs with fuel mixing provisions that even after liquid fuel has been "atomized" into fine droplets, the fuel is carried off by the air without being mixed. The conical shape of outwardly moving fuel droplets tend to make only surface contact with a stream of axially flowing volume of combustion air. For optimum combustion mixtures, it would be desirable to disperse the fuel uniformly throughout the combustion air. The dispersion process becomes more complex, when burner units are to be operated at non-constant energy output rates, as is desirable, when the burner units are used in aggregate drying and heating applications in the production of asphalt, for example. A further complication with respect to such aggregate drying and heating operations relates to the availability of fuel. Often the operators of the asphalt production equipment need to shift from one type of fuel supply to another. Fuel changes from coal dust to liquid fuel and to gaseous fuel need to be accommodated.
An optimization of one type fuel and air mixing system may have negative effect on another system for mixing air with another type fuel. The prior art shows various patents which seek to improve mixing of liquid fuel with combustion air.
U.S. Pat. No. 2,473,347 shows a central spray nozzle from which a conical shape of fine fuel droplets is emitted. An annular, axial air path is intersected by the conically outward proceeding spray of fuel. Thereafter the conically shaped fuel approaches a swirling volume of combustion air which volume is peripherally supplemented by secondary air which is also mixed through swirling vanes. The various volumes of air, except for that of the initial atomizing air are supplied at substantially the same pressure energy level and are combined in a swirling generally outward flow direction. Incomplete dispersion of the fuel into the various volumes of gas may result because the gases and the fuel progress at similar velocities and somewhat tangentially with respect to each other. Because of the speed at which combustion typically occurs after the fuel becomes mixed with primary air and is ignited, the mixture will be well toward the completion of the combustion process well before complete mixing can take place.
U.S. Pat. No. 4,526,322 seeks to improve the dispersion of fuel with initial combustion air by introducing fuel in a counterflow direction into a primary air stream. The impact of fuel and primary air is expected to enhance the mixing of the primary air with the fuel prior to the initiation of the actual combustion of the air.
U.S. Pat. No. 4,298,337 introduces liquid fuel which is atomized with high pressure air into a swirling primary air stream. An alternate gaseous fuel is distributed into the primary air by being routed together with the primary air through swirl plates.
U.S. Pat. No. 4,559,009 atomizes fuel oil by introducing the fuel oil in a continuous sheet into an inner stream of air and then collides the inner stream of air with an outer stream of air. The specification also discloses an annular pipe for introducing a gaseous fuel. The primary air intersects the annular axial flow pattern of the gaseous fuel to promote mixing.